The present invention relates to the field of operating a downhole tool string in a well. More particularly, the present invention relates to an improved system for providing electric power to downhole instruments and other tools and for transmitting electric signals from such tools to the well surface.
Downhole well instruments and other tools are positioned in hydrocarbon producing wells to detect well conditions and to control the operation of the well. Although fluid pressure communication systems have been developed to communicate between the well surface and downhole tools positioned at great depths below the well surface, such systems are affected by distortions within the well fluid and by delays in data transmission.
To avoid the problems associated with fluid based communication systems, metallic wire conductors communicate electric power and signals between the well surface and subsurface well equipment. In one example, U.S. Pat. No. 5,236,047 to Pringle et al. (1993) disclosed multiple well tools connected to a surface controller with multiple electric conductors. A separate electric conductor or signal line was connected between each downhole tool and the well surface. In a large installation having numerous downhole tools, the large space required by multiple hard wires and the plurality of wire failure paths restricts the usefulness of this concept.
The hard wires for communicating electricity to downhole tools are commonly referred to as I-wires. A separate I-wire is connected between well surface equipment and the downhole well tool. One type of tool comprises a pressure and temperature transducer, which typically produces one frequency for pressure and one frequency for temperature. A regulated voltage is transmitted through the I-wire from the well surface to the pressure and temperature transducer, and a linear regulator converts the voltage to the voltage required by the pressure and temperature transducers. Pressure and temperature signals are transmitted to the well surface with time division multiplexing techniques. The pressure signal from the pressure transducer is transmitted to the well surface by modulating the current in the I-wire. Similarly, the temperature signal from the temperature transducer is transmitted to the well surface by modulating the current in the I-wire when the pressure transducer is not transmitting.
This technique can be used for a single well tool such as the pressure and temperature transducer described. However, this technique has certain limitations. If another gauge or tool is positioned in the well, an additional I-wire must be installed. This increases the cost and requires the use of multiple I-wires in a wire array. Additionally, the electric signals can only be transmitted in one direction, such as from the downhole tool to the well surface. This limitation on the transmission of signals reduces the flexibility of the system for communicating information.
Certain limitations of the technique described above have been reduced in applications combining a pressure and temperature gauge and another tool such as a gas lift valve. In one example, a frequency band transmitted data from a pressure and temperature gauge, and a different frequency band transmitted data from the gas lift valve. To operate the gas lift valve, the current was reversed through the I-wire. To supply the substantially higher electric current required by the gas lift valve, the voltage was raised to overcome the current transmission losses caused by the resistivity of the I-wire. To prevent the high voltage from destroying the gauge, a diode was positioned in series with the gauge load to block the reversed line voltage.
Although this concept can permit a power and signal transmission on a single I-wire, the number of systems attachable to the I-wire is limited by the bandwidth of the I-wire. Moreover, the concept limits the commands to a single downhole tool at a time, which encumbers the ability to perform multiple simultaneous functions.
Accordingly, a need exists for a system for two-way transmission of signals and electric power between the well surface and downhole well tools. The system should be able to transmit different types of information and electric signals and should be able to meet different power requirements of downhole tools.